Terahertz Emission Spectroscopy Of Layered Molybdenum Disulfide

Two-dimensional?2D?materials have stimulated extensive attentions due to their remarkable physical,chemical,electronic,and photonic properties since the discovery of graphene.As a typical 2D material,layered molybdenum disulfide?MoS₂?possesses complementary yet distinctive properties compared with graphene,such as sizable energy structure and bandgap,high photoluminescence quantum yield of monolayer structure,ultrafast optoelectronic response with picosecond timescale,valley-contrasting optical selection rules,high exciton binding energy and so on.Thus,MoS₂ has been widely applied in transistor,photodetector,photovoltaic device,and light emission device.Since many physical and chemical processes of MoS₂ happen on surface and interface,the response and performance of MoS₂-based optoelectronic devices depend on the surface and interface properties.Terahertz?THz?emission spectroscopy is a sensitive and contactless spectroscopic tool for surface and interface property characterization with sub-picosecond timescale resolution and micro-to-nano spatial resolution,which carries rich spectroscopic information such as amplitude,waveform,phase,polarization,and polarity.Therefore,we have investigated the surface and interface properties of layered MoS₂ materials based on their THz emission spectroscopy and the analysis of physical mechanisms.The main works and innovation points are listed as follows:?1?The recent progresses of THz emission spectroscopy of graphene,transitional metal dichalcogenides?TMDs?,topological insulators,and hybrid perovskites are summerized.In contrast with traditional materials,these advanced materials demonstrate rich information at surfaces and interfaces,high nonlinear susceptibility,and unique response of surface or interface states since the physical and chemical properties are dominated by surface or interface states rather than bulk states.Thus,the investigation of THz emission spectroscopy of these advanced materials can not only promote the development of THz emission sources,but also pave a way for surface and interface characterization of advanced materials.This section has been published in Journal of Physics:Condensed Matter as a“Topical Review”.?2?The construction of THz spectroscopy system.We first demonstrated the setup of home-built THz spectroscopy system including THz time-domian spectroscopy,optical pump-THz probe spectroscopy,and THz emission spectroscopy based on femtosecond?fs?laser.Then we introduced the fs laser system,the construction process of THz spectroscopy system,the optimization of signal-to-noise ratio,electro-optical detection of THz signal,and data acquisition of THz components with perpendicular polarization.This section provides experimental basis for the THz emission spectroscopy of MoS₂ materials,which will help to deepen the understanding of the following works as well as offer a reference for other spectroscopic system construction.?3?THz emission spectroscopy of layered MoS₂ crystal in reflection configuration.We have investigated the surface property of MoS₂ crystal via THz emission spectroscopy,which indicates the dominant mechanism of THz radiation is surface-symmetry-breaking induced optical rectification.This is verified according to the radiated THz amplitude dependences on azimuthal angle,incident polarization angle,and pump fluence.We have also demonstrated the damage threshold of MoS₂ is⁵.66 mJ/cm² under the fs laser irradiation,and the microscopic bond breaking and formation are monitored via THz emission spectroscopy and Raman spectroscopy.The corresponding work is the first observation of THz radiation from TMDs,and the analysis of the second-order nonlinear polarization process will provide a theoretical reference for the performance of MoS₂-based nonlinear photonic devices.This work has been published in ACS Applied Materials&Interfaces.?4?THz emission spectroscopy of layered MoS₂ crystal in angle-resolved transmission configuration.The THz radiation contribution ratio of resonant optical rectification is⁹0%under normal incidence and decreases to⁴0%under-40^o incidence,which originates from the THz radiation contribution of the transient photocurrent induced by surface depletion field under oblique incidence.This surface field will lead to the THz radiation saturation due to the electrostatic screening effect by photoexcited carriers.Such pump dependent saturable THz radiation can be fitted well by the calculation from Maxwell equations with electromagnetic boundary condition.In addition,MoS₂ is diagnosed to be p-type from THz waveforms by comparison with GaAs.This suggests that THz surface emission spectroscopy can be used to determine the carrier type of samples based on the surface depletion field,which would be superior to conventional characterization of carrier type via Hall effect measurement as no contact electrodes are needed.This work has been published in The Journal of Physical Chemistry C.?5?THz emission spectroscopy of layered MoS₂ crystal in elliptically polarized configuration.Traditional spin splitting and spin polarization induced by spin-orbit coupling?SOC?can only happen via symmetry breaking.Herein,contrary to the expectations,we record spin polarization in centrosymmetric MoS₂ bulk crystal by helicity dependent photocurrent exposed as elliptically polarized THz emission.We assign this observation to SOC induced band splitting and optical selection rules under circularly polarized excitation.This spin polarization originates from the hidden SOC due to local Dresselhaus effect induced by the molybdenum atomic site asymmetry.Site asymmetry of Mo atom induces imbalanced distribution of photocarriers in k-space and generates circular photogalvanic current,which further results in elliptically polarized THz emission.Our results demonstrate the viability of all-optical control of hidden spin polarization in layered two-dimensional materials and propose applicability of the THz emission spectroscopy as a sensitive and contactless method to study spintronic physics in two-dimensional materials.This work has been submitted to Physical Review Letters.?6?THz emission spectroscopy of monolayer MoS₂ and graphene/MoS₂ heterostructure.Since the monolayer and bulk MoS₂ have different energy band structure,the photon energy of pump laser?1.55 eV@800 nm?is between the bandgaps of two MoS₂ structures?1.29eV and 1.9 eV?,which could result in different physical processes in monolayer and bulk MoS₂.Based on the THz emission spectroscopy with tunable wavelength,the primary mechanism of THz radiation from monolayer MoS₂ is nonresonant optical rectification under 1.55 eV excitation,while the transient shift current induced by linear photogalvanic effect dominates the THz radiation under 3.1 eV excitation,and the THz radiation amplitude is improved compared with that under 1.55 eV excitation.Moreover,the formation of a graphene/MoS₂ heterostructure can also enhance the THz radiation efficiency compared with monolayer MoS₂,which is due to the both contribution from the photon drag current of graphene and nonlinear polarization of MoS₂.Interestingly,shift current is generated in MoS₂ under 3.1 eV excitation,which will induce interface photocurrent flowing from MoS₂to graphene.The direction of such interface photocurrent is opposite from that under 1.55eV excitation,thus resulting in polarity flip of THz waveforms.This work provides a route for designing new THz emission sources based on heterostructure,and proposes THz emission spectroscopy to study the interface photocurrent as well as charge transfer dynamics of 2D heterostructures.Our research results are in the course of preparation for submission.